This invention relates to Squelch Circuits for radio receivers and in particular, to squelch circuits with automatic level control.
A squelch circuit can be defined as a circuit that causes the audio signal to be applied to an output device, such as a headset or speaker system, only during the reception of a modulated voice signal. An example of a prior art squelch circuit is disclosed in U.S. Pat. No. 3,102,236 and has as a distinguishing feature the ability to keep the squelch circuit from triggering, and activating the audio output device on a continuous tone. This is accomplished through the use of a syllabic rate filter rather than a two channel energy distribution comparison circuit which was the dominant method used in the prior art.
The prior art squelch circuits required that the operator of a radio receiver manually adjust the noise level control to either increase the sensitivity of the radio so that the desired audio signal may be passed to the audio output device or to increase the noise threshold to prevent noise from triggering the squelch circuit. Airborne radios are very susceptible to varying noise levels and consequently require a substantial amount of manual intervention for adjustment of each level as the aircraft in which these radios are mounted flies through different atmospheric conditions or changes from flying over land to flying over water. The change in the atmospheric conditions will at times, cause the prior art squelch circuits to switch on when there is nonintelligible information being broadcast, and thus distract the operator who in most cases is the pilot of the aircraft, and thus this action creates a safety hazard or lack of communications because of audio fatigue. Audio fatigue is defined as a condition where an operator as a result of prolonged listening to audio noise disables, "turns off," the radio receivers.